How to make this glass more realistic?
Refracting and reflecting materials need basically three things to look good:


Proper shape.


Light bounces all around and INSIDE an object. You need to pay attention how every single edge look like - for example just looking at the top edge of your glass I can say you need to give more definition to it. It should be sharper and inflated when compared to the side of a glass.

Also glass surface is never perfect (the only exception would be optical lenses when you are justified to model it as perfectly shaped objects). So what you do is just add a little bit of displacement to the glass, just enough to barely see that distortion effecting reflections and refractions... and you think it's barely visible, just divide it by half and usually that's your sweet spot. Unless of course you are aiming for roughly formed glass.

So first thing - grab a real glass and watch it as close as you can, then recreate every detail you can spot (which usually would be couple more edge loops and that slight displacement).


Proper lighting.


I can't think of a scene (except very very specfic ones) which looks good with one spot lamp. Try three point lighting at least, some high contrast lights or environment based lighting (rich surroundings that reflect and bounce light all around your scene or - which is best to start off - environment hdr light map).


Something to reflect.


Again - hdr map as a lighting source helps here, but if you can build a rich scene that fills the space it would be just as good, if not better. For a quick scene however your best bet would be hdr.

There are some specific shots with bright color and couple white light sources (like catalogue shots for glassware etc.) However those scenes are much harder arrange and they are actually target the synthetic looks of perfect and not very realistic look of clear glass. However if you look closely you still find that those shots still work because of proper setup and contrast.
And when rendering glass objects contast is very important, unless you aim for a milky, rough surface glass - you need very convincing highlights as your starting point and just the right amount of darker contrasting parts that will define your shape.

As an example you could look at these - not perfect or anything, but in the right direction I think:

Single area lightsource:


Two color high contrast light source: